Known optical telecommunication networks operate using Wavelength Division Multiplexing (WDM). To increase the total capacity towards higher line rates, e.g. transporting 400 Gb/s and 1 Tb/s, with higher spectral efficiency, has led to the introduction of the superchannel concept where the line rate is split into multiple subcarriers.
The most common approaches to implement superchannels are based on orthogonal signaling and Nyquist spacing, e.g. Orthogonal frequency-division multiplexing (OFDM) and Nyquist Wavelength Division Multiplexing (WDM). Both require subcarrier phase locking and precise frequency spacing. Therefore these solutions do not fit well with the currently installed networks where nodes comprise a fixed WDM grid e.g. with 50 GHz spacing. For example, the nodes are Reconfigurable Optical Add Drop Multiplexer (ROADMs) or optical cross-connects.
To overcome these difficulties the concept of flex-grid ROADMs is known, comprising a flexible grid for WDM channels. The spectral width of the channels can be tuned (e.g. in 12.5 GHz units) to carry the superchannel. This requires large changes to the network infrastructure.
An alternative approach is the use of higher order Quadrature amplitude modulation (QAM). The high spectral efficiency assists in fitting the subcarriers within the conventional 50 GHz slots. For example, a transmission of 224 Gb/s in a 50 GHz slot has been obtained with 16 QAM modulation. However QAM format is strongly penalized by its poor Optical Signal-to-Noise Ratio (OSNR) sensitivity which leads to poor reach (e.g. around 500 to 800 km).